(1) Field of the Invention
The present invention relates to a pulp produced from a polycarbodiimide, as well as to a process for production thereof. More particularly, the present invention relates to a polycarbodiimide pulp improved particularly in heat resistance, as well as to a process for production thereof.
(2) Prior Art
Polycarbodiimides have heat resistance, electrical insulating property, mechanical properties, reactivity, etc. and are therefore used in molding materials, hydrolysis-resistant agents, etc. Hence, it is anticipated that if a polycarbodiimide having such excellent properties could be made into a pulp, the pulp would find useful applications in various fields. In producing a pulp from a polycarbodiimide, it is considered to use a known process conventionally employed in production of an unwoven fabric or a pulp-like material made of a general-purpose thermoplastic resin, i.e., a splitting process, a polymer-blending process, a melt blowing process or the like. However, since polycarbodiimides are each a thermosetting resin and give rise to curing in the course of pulping, it is impossible to produce a polycarbodiimide pulp by any of the above processes.
Much effort was made in order to make an aromatic polyamide or the like into a pulp-like material, and many techniques were proposed. There can be mentioned, for example, techniques disclosed in Japanese Patent Publication No. 5732/1960, Japanese Patent Publication No. 13912/1963 and Japanese Patent Application Kokai (Laid-Open) No. 23602/1972. The techniques disclosed in these documents comprise discharging a dope of an aromatic polyamide in an organic solvent, into a coagulating bath (a liquid incapable of dissolving the aromatic polyamide) through a nozzle and vigorously stirring the bath to obtain a pulp-like material in the bath. Hence, it is considered to produce a polycarbodiimide pulp according to these techniques.
In the above techniques, the coagulating bath must be stirred uniformly and vigorously while a pulp-like material is produced continuously. Therefore, it is almost impossible from a practical standpoint to conduct vigorous stirring in a state that the shape and amount of solidified product change every moment, and uniformly apply a shear force to a dope introduced continuously, and therefore it is very difficult to obtain a pulp-like material of desired shape and quality.
Further, the polycarbodiimide dopes produced by conventional processes have insufficient stability and, in their pulping, give rise to gelling or becomes lumps owing to their severe agglomeration. Therefore, it is very difficult to obtain a pulp-like material from any of said dopes.
The conventional processes for producing a polycarbodiimide are disclosed in, for example, Japanese Patent Application Kokai (Laid-Open) No. 61599/1976; D. J. Lyman et al., "Die Makromol. Chem., 67, 1 (1963)"; and E. Dyeret al., "J. Am. Chem. Soc., 80, 5495 (1985)". In these processes, as the polymerization solvent, there are used hydrocarbons (e.g. benzene), o-dichlorobenzene, etc.; and as the organic diisocyanate, there are used 4,4'-diphenylmethane diisocyanate, tolylene diisocyanate and various other diisocyanates in a concentration of 10-25% by weight. In these known processes, the reaction system gives rise to gelling or precipitation in more than 10 minutes from the start of the reaction, making it impossible to obtain a reaction mixture in the form of a solution. Thus, the above conventional processes are unable to give a polycarbodiimide solution having excellent stability.
As a method for obtaining a polycarbodiimide solution having improved stability, there is a method of lowering the molecular weight of polycarbodiimide. This approach, however, is unable to give a pulp-like material and gives a powder of low heat resistance.
Meanwhile, the present inventor previously developed a polycarbodiimide pulp which is obtained by converting an organic diisocyanate into its quaternary ammonium salt and then converting the salt into a polycarbodiimide [Japanese Patent Application Kokai (Laid-Open) No. 62814/1991]. In view of the application purpose of polycarbodiimide, etc. it is desired to develop a polycarbodiimide pulp having improved heat resistance, etc.